Homeowners, commercial-property owners, and companies who provide care for lawns have an ongoing challenge of how to remove leaves, grass clippings, and other debris from lawns, gardens and paved surfaces, particularly when the debris accumulates to a depth of over an inch. Several companies now manufacture and sell combination vacuum-chipper-shredder machines intended for this purpose. Some U.S. patents illustrating the range of these machines are as follows: U.S. Pat. Nos. 5,931,396 and 5,799,365, both assigned to MTD Products; U.S. Pat. Nos. 5,381,970 and 5,231,827, both assigned to Garden Way Incorporated; and U.S. Pat. No. 5,642,864, assigned to Simplicity Manufacturing, Inc. However, the dilemma with many prior-art units is that while they are reasonably adapted to vacuum a thin layer of debris from the ground, they do not function well when there is a thick layer of leaves or other debris. When there is a covering of several inches of leaves, the collector duct merely pushes a stack of leaves ahead of the unit instead of drawing the leaves into the machine. If the collector duct is raised to be able to take leaves from the top of the overlayer, it is no longer so effective in collecting the leaves and other debris close to the ground. Accordingly, while the available machines are somewhat effective in removing the-thin layers of debris, the task of collecting thick layers of debris, particularly deep piles of leaves, remains quite problematic. Typical prior art machines, such as those shown in U.S. Pat. Nos. 5,799,365 and 5,231,827, have a vacuum pick-up opening wherein the plane of the opening is parallel to the ground, while the device shown in U.S. Pat. No. 5,642,864 has its opening disposed at an angle of about 45 degrees with respect to the ground. Such devices are poorly adapted to the tasks for which machine assistance is needed most.
Many existing machines depend primarily on flow of air to lift and entrain the leaves and other debris for transport through chipper shredder devices, and then to blow the reduced debris into accumulation units. To separate the debris from the air in which it is entrained, the machines rely on some form of filtration, with either fabric bags or screens. However, as leaf fragments and other debris collect on the filter, resistance to the flow of air increases, diminishing the air flow rate, and thereby, reducing the effectiveness of the vacuum pick-up opening.